Vertical shaft furnaces and methods for melting metals are well known in the art. Examples of such furnaces and methods are described in U.S. Pat. Nos. 4,311,519 to Berry; U.S. Pat. No. 4,844,426 to Barnes et al.; and U.S. Pat. No. 4,309,170 to Ward, all assigned to the assignee of this invention.
These prior apparatus and methods are directed to melting metals such as copper, aluminum, and aluminum alloys in a shaft furnace. In general, the prior art discloses a shaft furnace, a loading door through which the furnace is charged with the material to be melted, a bottom door, and a sloping hearth at the bottom of the furnace. Generally, the burners are positioned around the lower portion of the furnace so that melting takes place in that portion, with the material to be melted, or the charge, being loaded from above through the loading door. The charge works its way down the furnace and all the material which melts flows out a bottom door or taphole adjacent the hearth.
In one conventional design of a copper melting shaft furnace, a series of copper-blocks are arranged circumferentially about the interior wall of the furnace just below the loading door. Ambient air is admitted to a plenum surrounding these copper blocks to keep the blocks cool so that they do not melt. Copper blocks are advantageously used in this location so that when a scrap metal charge is introduced into the furnace through the charge opening or loading door, the copper blocks absorb the impact of the charge as it is loaded. If a refractory or a metal other than copper is used in this location, it is likely that the molten copper will become contaminated with such refractory or metal. However, because the blocks are made of copper, any particles or chips scraped or chipped off the blocks from the charge impacting thereon will not contaminate the melt.
While the combustion process in the metals melting furnace is complex and not completely understood, analysis of the process is possible on a theoretical basis. However, there are certain fundamental facts of furnace operation which provide a basis on which improved furnace construction and operation is possible. It is known, for example, that there is normally about 1.0% carbon monoxide (CO) in the combustion chamber of the furnace. Typically, there is somewhat less CO just above the top of the charge which has been loaded into the furnace because of partial burning of the CO with the air supplied to the furnace through the charge opening and with the cooling air supplied to the copper blocks, some of which leaks into the stack gases in the furnace. The conclusion that can be drawn is that hot CO in the presence of hot air will burn, or oxidize, without excessively cooling the combustion gases. However, if ambient dilution air is continuously admitted through the charge opening or an open loading door, for example, the air would excessively cool the hot stack gases below the temperature at which the CO will oxidize.
One prior art method for reducing CO emissions is to pass the furnace stack gases through a catalytic incinerator to burn all the remaining CO in the stack gases. Burners installed high in the stack and operating with excess oxygen are also used to burn off CO emissions.
In the case of a copper-melting furnace, molten copper has an affinity for oxygen so that it is typical to operate the furnace with a reducing atmosphere to minimize the pick-up of oxygen by the molten copper and thus minimize the oxygen content of the copper produced by the furnace. Accordingly, the burners are operated fuel rich to provide about 1.0 percent CO in the combustion chamber. This operating condition results in a molten copper from the furnace with an acceptable oxygen content of about 50-100 parts per million. This operating condition also allows substantial CO gas to escape into the atmosphere and, in recent years, this has become an important environmental concern.
In the case of an aluminum melting furnace normally aluminum is melted with an oxidizing flame. However, excess oxygen in the combustion chamber can result in ignition of the molten aluminum and formation of aluminum oxide particles which can be blown about in the furnace interior and potentially block the burner ports. Operation at a slight reducing atmosphere would minimize those problems, but will result in increased CO emissions.
As described above, one method of obtaining reduced CO emissions which has been tried in the past is to use a catalytic incinerator which is expensive to install and maintain. A catalytic incinerator includes a chemical or a metal which allows a combustion reaction to take place at less than normal combustion temperatures, for example, from about 414.degree. F. to about 900.degree. F. Placing extra burners in a furnace stack and operating them continuously with excess oxygen or air also allows burning of all the CO present. However, such an arrangement requires a continuous input of fuel and air to be operational and is uneconomical.
It would be desirable, therefore, to have the capability of operating a metal melting furnace, especially a furnace for melting copper metal, with a reducing atmosphere to avoid unnecessary oxidizing of the molten metal, yet, at the same time, operate the furnace in a condition with substantially no CO emissions.